1. Field of the Invention
The present invention relates to the constitution of an insulating gate type field effect semiconductor device (generally referred to as a thin film transistor or TFT) using a thin film semiconductor formed on an insulating substrate and a method of making thereof.
2. Description of Related Art
There has been a technology for making an active matrix type liquid crystal display device by forming a thin film transistor on an inexpensive glass substrate.
According to an active matrix type liquid crystal display device, TFTs are arranged to respective pixels of several millions which are arranged in a matrix, and electric charge inputted and outputted to and from each of pixel electrodes is controlled by a switching function of TFT.
The operational behavior of TFTs which are arranged at a pixel portion in a matrix, can visually be confirmed as a liquid crystal display. For example, in the case of a liquid display of normally black, portions where TFTs are not operated are represented as black spots in white display.
In this way, the malfunction of TFTs considerably deteriorates the appearance and therefore, high reliability is required to all of several millions of TFTs.
The problem here is the characteristic of an interface between a gate insulating film and a semiconductor film formed beneath the gate insulating film.
The deterioration in electric properties at the interface gives rise to cause a line defect in a liquid crystal display device and finally to cause malfunction in the display device. Therefore, promotion of the properties is needed.
Further, according to TFTs using a crystalline silicon film, if the crystalline silicon film constituting channel forming regions is intrinsic, the threshold value is generally shifted to rather negative (-) side from 0 V and the rise start voltage tends to be -2 through -4 V, in the case of a N channel type one. As a result, the tendency of constituting the normally ON state (a state rendered ON even if the gate voltage is 0 V) becomes conspicuous.
Under the normally ON state, for example, if TFTs are used as switching elements, current is made to flow even if the gate voltage is 0 V. Accordingly, in order to make a switch to be in the OFF state, the gate voltage needs to be always biased to the positive (+) side, and current consumption is increased in a circuit constituted by using the TFTs, or a circuit for applying the bias voltage may be needed.
In order to solve the problem, conventionally, even if TFTs of N channel type are formed, a threshold value control where a P-type impurity, for example, boron is doped to the crystalline silicon film comprising the channel forming region whereby the threshold voltage is shifted to the positive (+) side. As a result, TFTs in the normally OFF state (a state rendered OFF when the gate voltage is 0 V) can be formed. However, in order to carry out the threshold value control, a number of steps of manufacturing is increased which hampers reduction in manufacturing cost.